Field of the Invention
The present invention relates to a method of manufacturing a composite panel, a composite panel obtainable by such a method and to the use of a such a composite panel for the manufacture of rotor blades for wind energy systems, of panels for reefer containers or of panels for trailers.
Description of Related Art
The importance of wind energy is increasing steadily. This leads to an intense research into and further development of wind energy systems, especially into the rotor blades of such wind energy systems. A key consideration is the quality of the rotor blades and their economical production. Currently known rotor blades for wind energy systems are made from fiber reinforced materials based on resins as a matrix material, for example polyester resins (UP), vinyl ester resins (VE) or epoxy resins (EP). The production of the rotor blades is essentially the one-piece assembly of a lower side and an upper side which are placed on top of each other and bonded together. Stiffeners or belts may be included in the interior for added stability.
In this production of the rotor blade halves fiber reinforced composites are first prepared which need to harden. This hardening process is time-consuming and detrimental for a rapid overall production. Generally the rotor blades for wind energy systems using the aforementioned resins are made using techniques such as hand lamination, hand lamination with prepreg technology, wrapping or resin-assisted vacuum infusion. During hand lamination first a mold is prepared by applying a form release agent and optionally a gel coat onto the mold surface. Then glass fabrics with unidirectional or biaxial orientation are placed into the mold. A resin is applied and manually spread throughout the glass fabric by rollers. This step can be repeated as desired. Additional reinforcement elements such as belts or other components such as lightning protection can also be incorporated.
The hand lamination process using prepreg technology is similar to the hand lamination process. So-called prepregs (resin impregnated fiber mats) are prepared outside of the mold and are then placed into the rotor blade mold. This partial automation with respect to simple hand lamination processes have the benefit of a more constant production quality but workplace safety precautions must be applied when volatile organic compounds are involved.
In resin injection processes (also known as resin transfer molding (RTM), vacuum assisted resin transfer molding (VA RTM) or SCRIMP (Seemann composites resin infusion molding process)) the molds are prepared the molds are prepared by applying a release agent and optionally a gel coat. Then the dry fabrics are placed into the mold according to an exact plan. The layer which has been placed into the mold first will become the outermost layer of the product obtained such as a rotor blade.
Then a so-called distance layer is placed onto this first fiber reinforced layer. This distance layer is usually a balsa wood, polyvinyl chloride (PVC) foam or polyurethane (PUR) foam layer. Then a second (glass) fiber reinforced layer and optionally layers of other adjuvants are placed into the mold in an analog way as the first layer. The mold is sealed against vacuum and the air contained within the layers and distance materials is removed before resin is injected into the mold.
It is also possible that the elements of the distance layer also incorporate a (glass) fiber material. The production of these distance elements may be via a two-step wrapping process in which pre-fabricated foamed profiles are equipped with a (glass) fiber layer.
U.S. Pat. No. 3,544,417 is directed towards a cellular foam core assembly, the combination of: said assembly including at least one cellular foam core structure; each cellular foam core structure of said assembly including a series of generally parallel, longitudinally extending and generally transversely aligned foam cores, each core being preformed of a closed cell plastic foam material of a defined transverse cross section with all surfaces in transverse cross section being substantially flat, each core in transverse cross section having at least a lower base side and generally oppositely transversely, facing sides; each cellular foam core structure of said assembly including a primary base layer of fabric extending substantially continuously transversely and longitudinally along and fully contacting and covering said foam core lower base side, said primary base layer abutting said foam core lower base side and extending transversely between said foam cores substantially free of upward projection between said foam cores; each cellular foam core structure of said assembly including a primary covering layer of fabric extending substantially continuously upwardly over and downwardly transversely between said foam cores, said primary covering layer abutting all of said foam core oppositely transversely facing sides free of abutment with said foam core lower base slides and contacting said primary base layer at transverse extremities of each of said foam cores at said primary base layer completing said foam core longitudinal and transverse covering; each cellular foam core structure of said assembly including a generally longitudinally extending line of stitching at each of said foam core transverse extremities securing said primary base and covering layers together at said primary base layer and transversely between each transversely adjacent set of said foam cores of said each cellular foam core structure; said assembly including a lower secondary covering layer of fabric extending generally continuously transversely and longitudinally along fully contacting and covering said primary base layer of said at least one cellular foam core structure of said assembly, said lower secondary covering layer being free of upward projection transversely between any foam cores of said assembly; said assembly including an upper secondary covering layer of fabric extending generally continuously transversely being resin bonded thereto, said upper secondary cover and longitudinally along said assembly overlying and upwardly covering those parts of said primary covering layer of said at least one cellular foam core structure of said assembly at upper extremities of said foam cores thereof, said upper secondary covering layer being free of downward projection transversely between any foam cores of said assembly; said assembly including cellular foam core structure cores of said assembly being closely transversely adjacent one to the next transversely adjacent core with the primary covering layer of each core abutting the primary covering layer of that core's next transversely adjacent core; and cured resin covering, impregnating and rigidifying all of said base and covering layers of said assembly throughout said layers including said stitching, said cured resin bonding between said layers at said stitching and at all other areas of abutment between said layers rigidifying and bonding said assembly.
Reinforced foam cores are disclosed in U.S. Pat. No. 5,589,243. This patent relates to rigid foam boards and alternating absorptive fibrous web sheets which are adhered to form core panels or billets. Porosity is maintained in the webs for forming integral structural ties by absorbing resin applied to overlying sandwich panel skins. Beveled foam recesses adjacent web edge portions form structural resin fillets, and protruding edge portions form expanded connections to the skins Core panels oriented with their webs crossing are layered with web sheets to form enhanced panels. Boards or core panels and web sheets arranged in inclined stacks form second core panels having webs intersecting panel edges or faces at acute angles. Boards of alternating different physical properties and web sheets are bonded to form reinforced panels having differing interior and exterior densities. Compressible foam panels are used in place of or with web sheets to make bendable core panels. Foam between web edge portions is recessed for bonding a settable material to the edge portions.
U.S. Pat. No. 5,429,066 describes a composite structure and method of making the composite structure. A reinforcing fabric such as fiberglass is mechanically attached, for example, by stitching to a non-woven polyester fabric. The attached fabrics are placed in a mold with the non-woven fabric facing the inside of the mold. A self-expanding, self-curing foam is filled into the mold in an amount sufficient so that upon expansion in the closed mold, the foam penetrates into the interstices of the non-woven fabric which upon curing forms a bond therewith. The resulting structure can be used in a number of applications wherein the reinforcing fabric is later impregnated, for example, with a resin, and allowed to cure. Typical use of such a structure is as a stringer in fiberglass boat construction.
U.S. Pat. No. 5,908,591 is concerned with a method for making a composite structure, comprising the steps of: arranging a fabric layer in a configuration constrained against outward movement and defining a cavity between opposing surfaces of the fabric layer; dispensing a predetermined amount of a self-expanding, self-curable, uncured structural foam into the cavity, the foam expanding and curing in the cavity at a molding pressure determined by the predetermined amount of the foam and thereby attaching itself to the fabric layer to form the composite structure, the molding pressure causing the expanding foam to substantially fill only interstices of an inner portion of the fabric layer, without substantially penetrating an outer portion of the fabric layer; and, freeing the cured composite structure from the constraint of the arranging step, the outer portion of the fabric layer of the composite structure being thereafter substantially completely saturable with a curable material for lamination to another structure in a subsequent processing step. The method can further comprise the step of laminating the cured composite structure to another composite structure by saturating the outer portion of the fabric layer of the cured composite structure with a curable resin.
US 2002/0178992 A1 relates to a conformable composite reinforcing member which includes a cavity formed at least in part from a fabric layer and at least a first foam core and at least a second foam core positioned within the cavity. The second foam core has a relatively higher rigidity than the first foam core. The first foam core is preferably made from an open cell or flexible foam and the second foam core is preferably made from a rigid open cell foam.
WO 2009/102414 A1 discloses a fiber reinforced core panel having a first side and an opposing second side. The core panel contains a series of adjacent low density strips having at least three faces. The major face of each strip is disposed within the first or second side of the core panel and the major face of each strip is disposed within an opposite face of the core panel than the major face of the adjacent strips. The core panel also contains a continuous fibrous reinforcement sheet which is threaded through the low density strips such that the fibrous reinforcement sheet is disposed between adjacent strips and adjacent to the major faces of the low density strips. The reinforcement sheet forms at least about sixty five percent of the surface area of the first side and at least about sixty five percent of the surface area of the second side of the core panel.
EP 1 310 351 A1 discloses a method for making a windmill blade whereby problems with glue joints and with exposure of the workers to environmentally hazardous substances are avoided. This is effected by making the windmill blade in a closed mould with a mould core inside mould parts for formation of a mould cavity, in which fiber material and core material are placed. After applying vacuum to the mould cavity, matrix material is injected via a filling pipe, which is placed at a downwardly oriented side edge of the blade during the filling. Hereby is established a flow front which is used for indicating complete filling when this reaches the trailing edge of the blade and penetrates out through overflow apertures.
A further example for the use of layered composites is given in WO 2011/069975 A1 which relates to the use of layer superstructures in the production of rotor blades for wind power plants and to rotor blades for wind power plants.
The complexity of these processes is disadvantageous as it leads to higher production costs or a limited potential for automatic production of the sandwich elements.